There are two primary categories of continuous steel casting processes, i.e. “close casting” and “open casting”. In close casting processes, the use of a particular tube made from ceramic material, known as “submerged entry nozzle” (SEN), for conveying the liquid steel from the tundish to the mould allows most modern lubrication systems to be used, consisting of continuous casting powders. In open casting, where the cost due to the provision of the submerged entry nozzle is less, lubricating oils of mineral, vegetable or synthetic origin are typically used. This type of lubricant, however, does not always ensure effective lubrication. Consequently, there is excessive formation of scaling, cracks, lozenging and difficulty of lamination.
Casting powders used in close casting, normally consist of a mixture of various minerals. Such powders are available in various forms, for example, atomized granular powders, extruded powders and powders obtained by fitting. In terms of chemical composition, casting powders typically include a complex mixture of carbon, various oxides of mineral or synthetic origin (including SiO2, Al2O3, Na2O, CaO) and other materials.
There are four main functions carried out by lubricating powders, once added to the surface of the molten steel in the ingot mould, summarised as follows: i) heat insulation of the liquid steel in the CC mould, to prevent it from solidifying; ii) protection of the surface of the steel from oxidation; iii) lubrication and control of the heat exchange between the wall of the ingot mould and the outer shell of solidified steel; and iv) absorption of possible non-metallic inclusions coming from the steel. The powders, once poured into the ingot mould, lose part of the carbon by oxidation and heat up in contact with the liquid steel, forming a sintered layer and a molten one. The latter is distributed over the entire free surface of the steel and, thanks to the oscillations of the mould, infiltrates in the gap between it and the outermost shell of solidified steel. In this way, the liquid layer acts as a lubricant. The liquid that has infiltrated in turn partially solidifies in contact with the ingot mould, the wall of which is generally water-cooled, forming a layer of solid slag. The role of this layer is to allow an adequate level of heat transfer between the solidified steel shell and the ingot mould.
The casting powders make it possible to obtain a better quality steel, but they have the drawback of poor manageability, which makes them difficult to apply to open casting. In particular, the use of a casting powder is difficult to carry out due to particular feeding systems to be implemented with regard to the electromechanical, electronic and automation details.